The present invention relates to a method of and apparatus for applying liquid coating material onto a moving web of paper, and in particular to a coating method and apparatus of the fountain applicator type.
Coating a web of paper is generally effected by the application of a liquid coating material onto a moving web. The coating material may be comprised of a solid constituent suspended in a liquid carrier. The quality of the coating applied onto the paper web depends upon a number of factors, an important one of which being how the material is applied. The application of the coating material should preferably result in a coating that is continuous and uniform across the web.
One method previously used to coat paper webs was to feed liquid coating material to applicator rolls that applied the material directly onto the moving web. While the use of applicator rolls yields a fairly uniform coating across the web, as web speeds increase, there often occurs a film split pattern in the coating applied onto the web, i.e., cross-direction variations in the weight of the coating on the web. This technique therefore does not lend itself to coating webs at high speeds. Direct application by rolls also creates forces in the roll/web nip that imbed or force coating material into the web instead of covering the outer surface of the web to enhance smoothness.
In an attempt to avoid these and other problems, the art developed a coating process in which the liquid coating material was jetted in a free standing curtain of coating liquid directly onto the moving web with a fountain applicator. While fountain applicators overcome many of the limitations of roll applicators, in their use, skip coating often occurs. Skip coating is caused by air entrained in the coating liquid being contacted against the paper web and preventing the coating liquid from uniformly contacting and being uniformly applied onto the web surface. To decrease the severity of the skip coating problem, fountain applicators customarily include coating/air separation equipment to remove air from coating liquid prior to delivery of the coating liquid to the applicator, but the equipment is not 100% effective and some air remains entrained in the coating liquid jetted against the web surface and causes skip coating.